1. Field of the Invention
The present invention relates to methods using high power laser systems, and mirror systems for high power lasers based on Stimulated Brillouin Scattering SBS phase conjugation, to use of such lasers in master oscillator/power amplifier configurations, and to methods and systems for laser peening based on the same.
2. Description of Related Art
The use of mechanical shocks to form metals and to improve their surface properties has been realized for ages. In current industrial practice, a peening treatment of metal surfaces is accomplished by using high velocity shot. Treatment improves surface properties and very importantly for many applications, results in a part displaying significantly improved resistance to fatigue and corrosion failure. A wide range of components are shot peened in the aerospace and automotive industries. However, for many applications, shot peening does not provide sufficiently intense or deep treatment or cannot be used because of its detrimental effect on the surface finish.
With the invention of the laser, it was rapidly recognized that the intense shocks required for peening could be achieved by means of a laser-driven, tamped plasma. B. P. Fairand, et al., “Laser Shot Induced Microstructural and Mechanical Property Changes in 7075 Aluminum,” Journal of Applied Physics, Vol. 43, No. 9, p. 3893, September 1972. Typically, a plasma shock of 10 kB to 30 kB is generated at metal surfaces by means of high energy density (about 200 j/cm2), short pulse length (about 30 nanoseconds) lasers. A thin layer of metal tape, black paint or other absorbing material on the metal surface provides an absorber to prevent ablation of the metal. A confining or tamping material such as water covers the surface layer providing an increased intensity shock. These shocks have been shown to impart compressive stresses, deeper and more intense, than standard shot peening. In testing, this treatment has been shown to be superior for strengthening components from fatigue and corrosion failure. However, lasers with both sufficient energy and sufficient repetition rate to achieve production throughput at affordable costs have been difficult to provide.
One laser system which has been utilized for this purpose is described in our prior U.S. Pat. No. 5,239,408, entitled HIGH POWER, HIGH BEAM QUALITY REGENERATIVE AMPLIFIER. The laser system described in the just cited '408 patent comprises a high power amplifier in a master oscillator/power amplifier MOPA configuration capable of producing output pulses greater than 20 joules per pulse with the pulse width on the order of 20 to 30 nanoseconds or less using a wavefront correcting configuration based on a stimulated Brillouin scattering SBS phase conjugator/mirror system. The '408 patent refers to U.S. Pat. No. 5,022,033, entitled RING LASER HAVING AN OUTPUT AT A SINGLE FREQUENCY, as one implementation of a master oscillator. The oscillator geometry described in U.S. Pat. No. 5,022,033 produces very low energy pulses and therefore requires many more amplifier passes than is achievable with the amplifier system described in the '408 patent. In some applications, the master oscillator used in the system of the '408 patent was a standing-wave (2 mirror linear resonator) oscillator with an etalon output coupler. Another master oscillator configuration is described in our co-pending U.S. patent application Ser. No. ______, filed 30 October 2003, entitled SELF-SEEDED SINGLE-FREQUENCY SOLID-STATE RING LASER, AND SINGLE-FREQUENCY LASER PEENING METHOD AND SYSTEM USING SAME, which is incorporated by reference as if fully set forth herein.
A high power laser system such as that defined in the '408 patent, and in U.S. Pat. No. 5,689,363 “LONG-PULSE-WIDTH NARROW-BANDWIDTH SOLID STATE LASER,” employs a relay telescope for relaying images of the beam from the injection end of the system to the amplifier end and back. During the amplification process the beam passes through the relay telescope, passes through an amplifier, passes back through the relay telescope and then is routed via polarization rotation followed by reflection or transmission off of a polarizer element. During the polarization splitting process, the beam is not completely separated due to less than 100% rotation of the polarization and due to less than 100% separation (contrast) by the polarizer. The non-separated portion of the beam continues passing through the relay telescope and is re-amplified, often obtaining sufficient power so that it can become a damage problem elsewhere in the optical system. Stray or “ghost” reflections can also be amplified and need to be separated from the main beam. Some type of hardware setup is needed to allow alignment of the system, and then amplification and propagation of the desired high power beams while eliminating the unwanted beams.
It has been shown by the inventors that it is very important to place a relay-image of the distorted aperture of the amplifier at the input to the SBS phase conjugate mirror. This prevents the free optical propagation of the distorted beam which can cause phase aberrations that are introduced by the amplifier to be converted into non-uniformities in the spatial profile (irradiance distribution) of the beam. This is critical since an SBS phase conjugator very effectively reverses the optical wavefront of the input beam but often does not reproduce the irradiance profile with high fidelity. Therefore, wavefront errors that are converted to irradiance distribution errors may not be adequately corrected. In the relay-imaged system, the wavefront errors are accurately transported to the SBS phase conjugator. Nonuniformities introduced by imperfect irradiance reproduction in the nonlinear mirror are then minimized in the final amplifier passes due to gain saturation in the amplifier(s).
However, the multi-pass amplifier system can generate undesirable weak “ghost” beams that result from small shortcomings in the polarization control used for beam path switching. The amplifier optical train is designed so that these weak ghosts are emitted at slightly different angles from the main beam so that they should theoretically not interfere with the SBS phase-conjugation of the much more powerful primary beam. However, when the SBS mirror is operated at very high energies, well above its threshold, these weak beams can enter the cell and, even though they would be below threshold on their own, they can be efficiently reflected by the SBS mirror in a four-wave-mixing nonlinear interaction with the main input and output beams. Such beams can then cause damage to optical components in the system as they propagate within the amplifier.
An SBS phase-conjugated MOPA laser has very robust alignment characteristics since the lowest order aberration that is corrected by the phase-conjugate mirror is tilt. This means that the laser system can be very tolerant of small drifts in the precise alignment of optical components without causing a loss in output power or causing a re-pointing of the laser output. However, delivering the beam, propagating in the forward direction, to the SBS mirror must still be accomplished with some degree of precision. Prior art systems have required periodic monitoring and alignment adjustment by very skilled scientists. Thus, tools for simplifying the alignment of the system are needed.
An important issue in the operation of a high pulse energy, high average power, solid-state laser is preventing the possibility of internal optical damage to the amplifier due to nonlinear self-focusing of the amplified beam in the SBS medium. This is caused by the fact that the presence of a high optical irradiance inside the optical gain medium can cause small changes in the effective refractive index, an effect governed by the nonlinear refractive index of the material. Since this variation in index is correlated to small irradiance variations in the beam, these irradiance variations can grow until the beam profile breaks up into very small and very intense filaments, which damage the gain medium. This process is referred to as nonlinear self-focusing and can pose a serious limitation to the maximum pulse energy and peak power available from a solid-state laser.
For a laser processing application such as laser peening or peen forming, it is important that the laser pulses have the correct pulse duration, which depends on the type and thickness of material to be treated. In the high power, short pulse length, laser systems needed for these applications, controlling pulse duration is difficult. Techniques for controlling pulse duration are desirable for such systems.
In a laser peening application and in other applications of high power laser energy to target work pieces, a pulsed laser output is directed to a target for processing. Target surfaces are often comprised of reflective surfaces such as metal tape used in a laser peening application. In laser peening, the incident laser energy breaks down the target surface and rapidly forms a high temperature plasma. When fully formed, this plasma comprises a blackbody in that it is highly absorptive. However, during the early time portion of the pulse, the target surface is reflective and if its surface normal is oriented back along the laser optical axis, significant beam energy can be reflected back to the laser. This reflected light can damage the laser optics if allowed to propagate sufficiently far back up the axis. The laser beam is typically focused onto the target surface with a set of lenses used as target delivery optics, as described in U.S. Pat. No. 6,198,069, entitled “LASER BEAM TEMPORAL AND SPATIAL TAILORING FOR LASER SHOCK PROCESSING.” If the target surface were to be placed precisely at the focus of the lenses in the target delivery optics, the reflected beam would be returned with exactly reversed focusing characteristics and would match the incoming beam dimensions as it counter-propagates along the beam path. However, in most cases, the target is placed before the beam reaches focus to generate a required laser energy density in the desired spot size. In this situation, the reflected beam returns with different propagation characteristics than the incoming beam. This can cause the reflected beam to come to focus at undesirable locations in the optical beam train such as at the surfaces of critical optical components such as lenses and mirrors. This can result in permanent damage to these critical components in the optical beam train. Further, the local shape and curvature of the target can add to the focusing characteristics of the reflected beam, resulting in unexpected focused hot spots in the reflected beam. Finally, laser peening generally uses a flowing transparent liquid layer (such as water) over the treated surface. Small ripples and non-uniformities in the surface of the water can also result in beam distortion in the reflected beam and unexpected focusing characteristics. A means is needed to significantly reduce laser energy reflected back from the target to prevent damage.